1. Field of the Invention
The present disclosure is generally related to communication systems and methods and, more particularly, is related to collision avoidance systems and methods in a wireless network.
2. Related Art
Communication networks come in a variety of forms. Notable networks include wireline and wireless. Wireline networks include local area networks (LANs), DSL networks, and cable networks, among others. Wireless networks include cellular telephone networks, classic land mobile radio networks and satellite transmission networks, among others. These wireless networks are typically characterized as wide area networks. More recently, wireless local area networks and wireless home networks have been proposed, and standards, such as Bluetooth and IEEE 802.11, have been introduced to govern the development of wireless equipment for such localized networks.
A wireless local area network (LAN) typically uses infrared (IR) or radio frequency (RF) communication channels to communicate between portable or mobile computer terminals and access points (APs) or base stations. These APs are, in turn, connected by a wired or wireless communications channel to a network infrastructure which connects groups of access points together to form the LAN, including, optionally, one or more host computer systems.
Wireless protocols such as Bluetooth and IEEE 802.11 support the logical interconnections of such portable roaming terminals having a variety of types of communication capabilities to host computers. The logical interconnections are based upon an infrastructure in which at least some of the terminals are capable of communicating with at least two of the APs when located within a predetermined range, each terminal being normally associated, and in communication, with a single one of the access points. Based on the overall spatial layout, response time, and loading requirements of the network, different networking schemes and communication protocols have been designed so as to most efficiently regulate the communications.
IEEE Standard 802.11 (“802.11”) is set out in “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications” and is available from the IEEE Standards Department, Piscataway, N.J. The IEEE 802.11 standard permits either IR or RF communications at 1 Mbps, 2 Mbps and higher data rates, a medium access technique similar to carrier sense multiple access/collision avoidance (CSMA/CA), a power-save mode for battery-operated mobile stations, seamless roaming in a full cellular network, high throughput operation, diverse antenna systems designed to eliminate “dead spots,” and an easy interface to existing network infrastructures. The IEEE Standard 802.11b extension supports data rates up to 11 Mbps.
One problem that may occur in wireless LAN (WLAN) systems involves the use of extended interframe space (EIFS). As is known, EIFS is a time during which backoff is suspended, which is long enough to allow for transmission of an acknowledgment (ACK) frame from a hidden node. In general, an EIFS is started when a PHY header is received correctly, but the MAC header is not (e.g., the frame check sequence (FCS) failed). If the ACK transmission is not actually hidden in the physical sense (e.g., the PHY header can be received) but the MAC portion cannot be decoded, the station starts a new EIFS at the end of the ACK transmission, while in practice it should have continued its backoff at that time. Furthering this problem is the fact that the ACK transmission may be much shorter than the EIFS duration (which is based on the lowest mandatory PHY rate), especially in the 802.11g standard where EIFS is 320 microseconds and an orthogonal frequency division multiplexing (OFDM) ACK transmission plus short interframe space (SIFS) may be as short as 34 microseconds. Such disparity in durations introduces a significant unfairness for nodes which start an EIFS versus those nodes which can decode the MAC portion of the frame and hence continue backoff immediately after the ACK transmission.
Another problem that may occur in WLAN systems involves the resetting of a network allocation vector (NAV) by clients, through the transmission of a CF-End frame at the end of a transmit opportunity (TXOP). One issue that may arise when a client resets a NAV by transmitting a CF-End frame is that this CF-End frame may not be received by all nodes in the basic service set (BSS) because the client may not be near the center of the BSS. In this case, the NAV is reset only in part of the BSS.